To make a valve closure action quick of a main valve opening and closing a passage between a water pressure source unit and an injection valve.

The main valve 10 arranged in a main passage 16 between the water pressure source unit 2 and the injection valve 3 includes a main body 20 having a water pressure source unit side port 24 and an injection valve side port 25, a valve seat 21 formed between the both ports, a valve element 22 capable of seating on the valve seat 21 and opening and closing the main passage 16, and a back pressure chamber 23 for energizing the valve element in a seating direction. A first solenoid valve 1 opens and closes an introduction passage 17 introducing working fluid to the back pressure chamber 23, and a second solenoid valve 12 opens and closes a discharge passage 18 discharging working fluid from the back pressure chamber 23. A pressure compensation mechanism 34 maintains pressure of working fluid in the back pressure chamber 23 when working fluid is discharged from the back pressure chamber 23 at predetermined pressure which is lower than water pressure acting on the water pressure source unit side port 24 and is higher than pressure of working fluid in a tank 28.

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